The present invention relates to a process for the hermetic adhesive bonding of plastic casing parts of a battery or of a galvanic cell.
German book publication: "Taschenbuch der Kitte und Klebstoffe" (Pocketbook of cements and adhesives), Dr. E. Plath, L. Plath; Wissenschaftliche Verlagsgesellschaft mbH, Stuttgart (1963), pages 328-29, describes a process for adhesive bonding of plastic parts made from polyamide, in which the joining surfaces of the parts are thinly coated with formic acid, dried slightly and then pressed together, as well as an apparatus for carrying out the process. In some cases of batteries or galvanic cells, however, it is necessary to keep the leakage rate of the adhesion very low. For instance, in the case of gas-tight nickel/cadmium cells with oxygen consuming electrodes, it is necessary to achieve a tightness which allows a leakage rate of no more than 10.sup.7 mbar.times.liter per sec. (referred to as 50 mbar helium), in order to ensure a service life of many years. Otherwise, the ingress of the nitrogen contained in the atmosphere would considerably impair the gas kinetics, and consequently the functioning of the cell.
Since polyamide 6 is generally used as plastic for the casing, it is usual to seal such a gas-tight cell by diffusion bonding by means of formic acid. This type of bonding, and design tips for the bonding seam, are described in VDI codes of practice; "Kunststoffkleben" (adhesive bonding of plastics); VDI 3821. Generally, quite good results are obtained in such adhesive bonding of the cell casing, but there is a relatively high rejection rate with respect to the required tightness due to the partial dissolution of the polyamide 6 by the formic acid, which causes the formation of many small gas passages in the region of the joining surfaces (microporosity) thereby increasing gas permeability. Moreover, the application of the adhesive solution, which is toxic and very aggressive, is complicated, time-consuming and thus expensive.
The object of the present invention is to develop a process by which the rejection rate is lowered and the adhesive bonding is made easier in terms of the process and in terms of design.
This object is achieved by the process according to the invention, in which the plastic casing of the future battery or galvanic cell is produced in an efficient way, the bonding region having a low to insignificant microporosity. As a result, on the one hand the rejection rate is lowered in production and on the other hand the tightness of the bond is ensured.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.